The structure and replication of avian and murine tumor viruses will be studied. The key questions to be studied about RNA tumor viruses include: (1) Physical analysis of the viral RNA of different classes of RNA tumor viruses: Are the recently discovered distinctions between the RNA's of some transforming and nontransforming viruses generally valid? (2) Analysis and comparison of the viral type-specific glycoproteins: Does the virus, the host, or do both control glycosylation of viral envelope proteins? (3) Replication of viral RNA: Is the virus-associated DNA polymerase the replicase? This question will be approached by analysis of the in vitro properties of the enzyme. Thereafter, the role of the enzyme in the cell will be studied. The search for DNA or RNA intermediates by hybridization techniques will be continued. (4) Structure of the so-called nucleoid of Rous sarcoma virus will be studied and compared to the classical nucleocapsids of cytocidal viruses. (5) Also influenza virus, a cytocidal RNA virus is investigated. Influenza virus will be used for comparison with Rous sarcoma virus, both with regard to the structure of the two viruses and their replication. Both influenza virus and Rous sarcoma virus have a subunit structure of their RNA. Both show an unexplained sensitivity to actinomycin D. Both viruses will be grown in chick embryo fibroblasts if a close comparison is desired. Further, it will be examined by competitive hybridization whether or not the distinct RNA's of influenza virus have distinct base sequences.